Batteries can be broadly classified into primary and secondary batteries. Primary batteries, also referred to as disposable batteries, are intended to be used until depleted, after which they are simply replaced with one or more new batteries. Secondary batteries, more commonly referred to as rechargeable batteries, are capable of being repeatedly recharged and reused, therefore offering economic, environmental and ease-of-use benefits compared to disposable batteries.
Although rechargeable batteries offer a number of advantages over disposable batteries, this type of battery is not without its drawbacks. In general, most of the disadvantages associated with rechargeable batteries are due to the battery chemistries employed, as these chemistries tend to be less stable than those used in primary cells. Due to these relatively unstable chemistries, secondary cells often require special handling during fabrication. Additionally, secondary cells such as lithium-ion cells tend to be more prone to thermal runaway than primary cells, thermal runaway occurring when the internal reaction rate increases to the point that more heat is being generated than can be withdrawn, leading to a further increase in both reaction rate and heat generation. Eventually the amount of generated heat is great enough to lead to the combustion of the battery as well as materials in proximity to the battery. Thermal runaway may be initiated by a short circuit within the cell, improper cell use, physical abuse, manufacturing defects, or exposure of the cell to extreme external temperatures.
During a thermal runaway event, a large amount of thermal energy is rapidly released, heating the entire cell up to a temperature of 900° C. or more. Due to the increased temperature of the cell undergoing thermal runaway, the temperature of adjacent cells within the battery pack will also increase. If the temperature of these adjacent cells is allowed to increase unimpeded, they may also enter into a state of thermal runaway, leading to a cascading effect where the initiation of thermal runaway within a single cell propagates throughout the entire battery pack. As a result, power from the battery pack is interrupted and the system employing the battery pack is more likely to incur extensive collateral damage due to the scale of thermal runaway and the associated release of thermal energy.
A number of approaches have been employed to either reduce the risk of thermal runaway, or reduce the risk of thermal runaway propagation. For example, by insulating the battery terminals and using specifically designed battery storage containers, the risk of shorting during storage and/or handling can be reduced. Another approach is to develop new cell chemistries and/or modify existing cell chemistries. Yet another approach, disclosed in co-pending U.S. patent application Ser. Nos. 12/504,712, 12/460,372, 12/460,342, 12/460,423 and 12/460,346, is to provide additional shielding at the cell level, thus inhibiting the flow of thermal energy from the cell undergoing thermal runaway to adjacent cells. Still yet another approach, disclosed in co-pending U.S. patent application Ser. No. 12/545,146, is to use a spacer assembly to maintain the position of the battery undergoing thermal runaway in its predetermined location within the battery pack, thereby helping to minimize the thermal effects on adjacent cells.
Accordingly, what is needed is a system that monitors the thermal behavior of the cells within a battery pack and, once thermal runaway is detected, mitigates the effects of this event in an attempt to prevent the initial thermal runaway event from propagating to neighboring cells within the battery pack. The present invention provides such a system.